Cuyahoga County Sheriff Clifford Pinkney said a "majority" of his department's investigation of the November killing of 12-year-old Tamir Rice by Cleveland police was finished, though he did not offer a deadline for its completion.

Pinkney's department began its work in January when Cleveland police and city government officials agreed to hand over the investigation to the sheriff's office.

"We have been tirelessly working on this investigation," said Pinkney, who did not take questions, citing the ongoing investigation.

The lack of answers from Pinkney prompted some attending the news conference to shout as it concluded, "Six months, it's been six months!"

Tamir's death, along with the killings of Michael Brown in Ferguson, Mo. and Eric Garner in Staten Island, N.Y., helped stoke a national conversation about race and policing. All three were black and unarmed when killed during confrontations with police; Brown and Garner were adults.

The investigation of the boy's death has taken considerably longer than the reviews of Garner's and Brown's killings.

A St. Louis County grand jury in late November declined to indict Ferguson Police Officer Darren Wilson in Brown's shooting, a little more than three months after the 18-year-old was shot. In early December, a Staten Island grand jury declined to indict a New York police officer in Garner's death, 4½ months after the 43-year-old died while held by an officer in what many have called a chokehold.

Law enforcement experts cautioned the public against comparing the cases.

“I would think that six months is sufficient to gather information for a grand jury, but there may be facts involved in this case that you and I are not aware of," said Wayne Fisher, a professor of police policy at Rutgers University in New Jersey. "It’s very difficult to say all cases should reach the grand jury on a certain fixed timeline.”

Cleveland City Councilman Jeff Johnson said he understood residents' concerns, but also counseled patience.

“Justice is slow. Sometimes it should be criticized," Johnson said. "In this case, when it comes to Sheriff Pinkney, I am comfortable with how he’s handling that.”

After Pinkney's department completes its investigation, it will deliver evidence to the county prosecutor's office, which then will present the case to a grand jury. It will be up to the grand jury to decide whether either of the officers involved in the shooting should face criminal charges.

Joseph Frolik, a Cuyahoga County prosecutor's office spokesman, couldn't say how long it would take his office to present the case to a grand jury after the Sheriff's Department turns over its evidence.

Tamir was shot and killed Nov. 22 by police officers responding to a report of a man brandishing a gun. The caller told the 911 dispatcher that the person may have been a juvenile and that the gun was "probably fake," but that information was not relayed to the responding officers.

Video of the shooting shows rookie Officer Timothy Loehmann, 26, opening fire almost immediately after his police cruiser came to a stop near the boy. Police officials have said Loehmann and his partner, Frank Garmback, 46, believed Tamir had a firearm and was advancing toward them.

The shooting sparked citywide outrage and caused many to question the qualifications of Loehmann, who had been fired from another Ohio police department for poor performance and possible emotional instability.

Tamir's death, which along with the Garner and Brown cases, among others, has driven a wedge between law enforcement agencies and the people they are sworn to protect, added to the troubles facing Cleveland police.

Less than a month after the shooting, the federal Department of Justice completed an investigation of the city Police Department and found the agency routinely used excessive force and sometimes used chemical sprays and stun guns to punish, rather than subdue, suspects.

That review was, in part, sparked by the killing of two people in a wild police pursuit that involved more than 100 officers in 2012. Cleveland Police Officer Michael Brelo was charged with manslaughter in that incident and his trial is ongoing.